I got my 'Protect-a-Bed' matress protector at Home Outfitters. They're kinda pricey, but it's worth it to not have to replace a mattress ever again. They are completely waterproof, but it's not like having a rubber sheet on your bed, it's like a membrane on the underside and terry cloth on top. The other bonus is that the cover actually prolongs the life of the mattress by preventing sweat and dirt, mites, allergens etc... from getting in the mattress.

IKEA also sells mattress protectors much cheaper, but I'm not sure if they are as waterproof as the 'Protect-a-Bed'.